pkg-config support insufficient for certain static linking scenarios

**Describe the bug**
Cabal unconditionally invokes `pkg-config --libs` when collecting arguments, which is not sufficient for certain static linking scenarios:

https://github.com/haskell/cabal/blob/00a2351789a460700a2567eb5ecc42cca0af913f/Cabal/src/Distribution/Simple/Configure.hs#L1640

When statically linking, it should use `pkg-config --static --libs` instead.

**To Reproduce**
1. Download [`hw-kafka-client`](https://github.com/haskell-works/hw-kafka-client) and change its `extra-libraries: rdkafka` to `pkgconfig-depends: rdkafka`.
2. Create a package with `build-depends: hw-kafka-client`, use a `cabal.project` file to have the test package use your local `hw-kafka-client` and attempt to build it using a musl64 cross-toolchain (easiest to use `haskell.nix` for this). Expect to see unresolved symbol errors of the form:

```
/nix/store/b07pcb91l2q1iasnqj74bajikd43b714-rdkafka-1.6.0-x86_64-unknown-linux-musl/lib/librdkafka.a(rdkafka_ssl.o):function rd_kafka_ssl_ctx_init: error: undefined reference to 'OpenSSL_version_num'
```

(The failure can be worked around with `extra-libraries: rdkafka ssl crypto z`.)

Cause of failure:
* The linker command line mentions `-lrdkafka` but not `-lssl -lcrypto`, which are required by objects in `librdkafka.a`.
* `rdkafka.pc` correctly lists the `-lssl` etc in `Libs.private`.
* Reading between the lines of the [`pkg-config` manual](https://people.freedesktop.org/~dbn/pkg-config-guide.html#writing), the `Libs.private` are only added when statically linking. You can confirm this by running `pkg-config --static --libs rdkafka` and `pkg-config --libs rdkafka` and comparing their output.
* Cabal only ever invokes `pkg-config --libs` as far as I can tell (see link at top of issue), so the musl64 build fails.
A package that has `build-depends: hw-kafka-client` will not statically link (using `musl64` cross-toolchain from `haskell.nix`) due to missing `-lssl` and `-lcrypto` on the linker command-line.

**Expected behavior**
Additional flags for static linking should make their way through to the linker command line, and the program should build without errors.